1. Field of the Invention
This invention relates to an orthodontic appliance and a method for manufacturing an orthodontic appliance that is made of polymeric material and reinforcing fiber embedded in the polymeric material.
2. Description of the Related Art
Orthodontic treatment concerns movement of improperly aligned or malpositioned teeth to desired orientations. Orthodontic treatment often involves the use of small appliances known as brackets that are secured to incisor, bicuspid and cuspid teeth, and buccal tubes that are secured to molar teeth. The brackets and buccal tubes have small slots or passages that receive a resilient, metallic archwire. The archwire forms a track to guide movement of the teeth, and the teeth are urged toward desired positions by bends or twists placed in the archwire, or by the use of other devices such as elastomeric modules.
Many brackets and buccal tubes have been made of stainless steel because stainless steel is strong, non-absorbent, weldable and relatively easy to form and machine. However, adults and older children undergoing orthodontic treatment are sometimes embarrassed by the "metallic smile" appearance of such brackets. Further, certain patients are allergic to nickel and chromium that are often present in stainless steel appliances.
Orthodontic appliances are sometimes made of materials other than alloys containing nickel and chromium and can be used for patients sensitive to these elements. For example, orthodontic appliances may be made of a ceramic such as monocrystalline or polycrystalline alumina. U.S. Pat. No. 4,954,080 describes orthodontic appliances made of a polycrystalline ceramic having a translucency which minimizes visibility of the appliance when mounted on a tooth so that the problem of a "metallic smile" is largely avoided.
However, ceramic brackets are known to present a greater resistance to movement of the bracket relative to the archwire as compared to metal brackets. Resistance to movement is considered a disadvantage because the resistance slows movement of the teeth to positions desired by the orthodontist and can lengthen treatment time.
Orthodontic appliances have also been made of a plastic material that in some instances is translucent and neutral in color. Preferably, the selected plastic material is relatively resistant to staining by food and beverages such as mustard, spaghetti sauce and grape juice. It is also desired that the plastic material be relatively resistant to creep so that the sides of the archwire slot do not unduly deform.
Attempts have also been made to strengthen plastic orthodontic appliances by use of reinforcing fibers or whiskers. For example, plastic brackets containing fibers were promoted by American Orthodontics of Sheboygan, Wisconsin as early as 1985. U.S. Pat. No. 5,078,596 discloses an orthodontic bracket made of polycarbonate and translucent glass or ceramic fibers. It has been suggested that brackets may be made by injecting both resin and chopped fibers into a mold assembly.
U.S. Pat. No. 4,717,341 describes an orthodontic bracket produced from glass-filled polycarbonate composite material by injection into a heated aluminum mold. An opening to the mold is placed in the base directly under the center of the slot apparently in an attempt to achieve a certain orientation of the fibers.
There is a continuing interest in making orthodontic appliances as small as practicable so that for aesthetic reasons the visibility of the appliance is reduced. However, it is important that the appliance have sufficient strength to resist breakage during ordinary use. Orthodontic appliances are often subject to significant stress during mastication when the bracket may contact hard food portions, occluding teeth or appliances mounted on occluding teeth.
In addition, certain smaller sections of appliances need to withstand relatively strong forces that may be intentionally induced during orthodontic treatment. For example, brackets often have tiny tiewings shaped like hooks on opposite sides of the archwire slot for receiving a wire or elastic ligature that is used to bind the archwire to the bracket. Such tiewings, if weak, may break apart from the body of the bracket when an attempt is made to firmly seat the archwire in the bottom of the slot. As can be understood, replacement of fractured brackets is a nuisance for both the orthodontist and patient.
There is a continuing need in the art for a plastic orthodontic appliance that is relatively small and yet has sufficient strength and stiffness, particularly in regions of the tiewings and sections near the archwire slot. Preferably, such a bracket would be made according to a method that is relatively inexpensive, adaptable for automation and yet provides consistent, satisfactory results.